A hundred years ago it was widely believed that there was life on Mars. The American astronomer Percival Lowell even produced detailed maps of canals he claimed had been constructed by water-deprived Martians. Then in the 1960s space probes sent to Mars failed to reveal any sign of life, let alone intelligent canal-building life. But the coup de grace came in 1977 when the US space agency Nasa landed two Viking spacecraft on the Martian surface with the specific aim of searching for signs of biological activity. Not so much as a bacterium was found. The surface of Mars appeared to be a freeze-dried desert, utterly hostile to any form of life.

Today this pessimistic assessment seems too hasty. I believe not only that Mars has harboured life, but it may actually be the cradle of life. This conclusion arises because of the recent discovery that our biosphere extends deep into the bowels of the Earth. Microbes have been found thriving at depths of several kilometres, inhabiting the pore spaces of apparently solid rock. Genetic studies suggest these deep-living organisms are among the most ancient on the planet. They are, in effect, living fossils.

Because temperature sharply rises with depth, the subterranean microbes tend to be extremely heat-tolerant. There is, however, a limit. Estimates suggest that 150C is probably an upper bound for life as we know it. After Earth formed about 4.5bn years ago it remained very hot, both from enhanced radioactivity and the violence of the planet's birth. Temperatures below ground would have been lethal, even for heat-loving microbes. On the other hand the surface was pretty uncongenial too. Astronomers think that for about 700m years a barrage of giant asteroids pounded the planet. The big impacts would have swathed the globe with incandescent rock vapour, boiling the oceans and sterilising the rock beneath.

By contrast, Mars cooled quicker because it is smaller. The comfort zone for deep-living, heat-tolerant microbes would have been deeper sooner. All in all, the Red Planet offered a more favourable habitat for life during the early history of the solar system. We don't know where life began, but a kilometre or two below the surface of Mars seems a good place. How, then, did life get from Mars to Earth? The answer is straightforward. The same asteroid impacts that made early life so hazardous also served to splatter vast quantities of Martian rock around the solar system. A fraction of this hits Earth; indeed, it does so today. So far, a couple of dozen meteorites have been found that can be traced back to Mars.

If there was life on Mars, then it is possible that some Martian microbes will have hitched a ride inside the ejected rocks and made their way to Earth. When I suggested this idea about 10 years ago, few scientists took it seriously. They found it incredible that any form of life could survive being blasted off a planet and subjected to the inhospitable environment of outer space. Yet evidence is steadily growing that microbes could withstand the violence of ejection, the savage radiation of interplanetary space, as well as the heat of atmospheric re-entry. Studies of the Martian meteorites show they were not highly shock-heated when propelled into space. As for the microbes, cocooned inside rocks a metre or more across, they would be shielded from the worst effects of radiation.

Initially Mars was the more bio-friendly planet; Earth was a scalding hell. Once life got going on the Red Planet, it quickly spread through the subsurface zone - a good refuge from impacts. However, those microbes living near ground zero of a major impact would have been flung into orbit round the sun. The lucky ones, buried deep inside large boulders, could have survived in space for millions of years. A few of those boulders would, over such durations, hit the Earth. Although many microbes would perish in space, and more would die on high-speed entry to Earth's atmosphere, it would take just one viable organism to seed our planet with life.

One of the puzzles about life's appearance on Earth is that it happened so quickly after the bombardment abated about 3.8bn years ago. There are distinct traces of life in Australia dating from 3.5bn years ago, and hints of life in rocks from even earlier times. This is readily explained if life came from Mars. We can imagine a continuing rain of microbe-laden Martian debris falling on Earth during the bombardment. As soon as conditions finally settled down, these colonists would have flourished. Martian life probably established itself here many times, only to be destroyed by the next big impact. If I am right, then you and I are the direct descendants of the first Martians able to burrow hot and deep, and ride out the remaining fury of the cosmic bombardment.

·Paul Davies is a member of the Australian Centre for Astrobiology in Sydney and a visiting professor at Imperial College. His book The Fifth Miracle: the Search for the Origin of Life is published by Penguin.